1. Field of the Invention
The present invention relates to a three-wave antenna for vehicles used for transmitting and receiving three types of waves including transmit-receive waves for car telephone (hereafter abbreviated as "MT waves"), amplitude modulation broadcast waves (hereafter abbreviated as "AM waves"), and frequency modulation broadcast waves (hereafter abbreviated as "FM waves").
2. Prior Art
Conventional vehicle antennas are mainly used for receiving AM/FM waves. Therefore, a single unit sharing a two-wave (AM/FM) antenna is designed to receive these two types of waves. Over recent years, however, the widespread use of vehicle telephones has made it increasingly necessary to install antennas which transmit and receive MT waves. An MT wave (usually, ultra short waves) is far different from AM/FM waves in frequency band and also in radiation characteristics. Accordingly, MT wave antennas have conventionally been installed separately.
When an MT wave antenna is installed separately from the AM/FM wave antenna, it has the following disadvantages. Firstly, from an aesthetic standpoint the appearance of vehicle is adversely effected. In addition, other disadvantages include the high manufacturing cost of such antennas and complex work required for installation. Accordingly, there is a simple solution to such problems and that is to combine the MT wave antenna into a single unit with the AM/FM wave antenna.
To this end, the inventors of the present invention have invented a three-wave vehicle antenna in which an insulating pipe is connected to an end of a conductive pipe; an MT wave antenna element and a matching coaxial MT wave device are housed in the insulating pipe; one end of a coaxial feeder cable, with its other end connected to the matching device, is led into the body of the automobile through the conductive pipe; and the pipe is used as an AM/FM antenna element.
In the above-described three-wave vehicle antenna, the antenna is unified and the foregoing problems are solved. However, other problems still remain. The coaxial MT wave feeder cable passes through the inside of the conductive pipe as the AM/FM antenna element. Accordingly, due to the stray capacity present between the outer conductor (braided shielding wire) composing the feeder cable and the conductive pipe, the sensitivity of the AM/FM antenna is lowered. In order to prevent such a decrease in sensitivity, the gap between the inner circumferential surface of the conductive pipe and the outer circumferential surface of the feeder cable must be widened. However, when this is done the conductive pipe becomes larger in diameter, thus making the whole body of the antenna larger in size.
Accordingly, the object of the present invention is to provide a three-wave vehicle antenna which reduces the stray capacity between the conductive pipe and the coaxial MT wave feeder cable, without causing the conductive pipe to be increased in diameter and is thus capable of preventing the sensitivity of the AM/FM wave antenna from being lowered.
The above object is achieved via a unique structure for an antenna wherein the antenna is constructed such that an insulating pipe is connected to the tip of a conductive pipe, with a MT wave antenna element and a coaxial MT wave matching device housed in the insulating pipe. One end of the coaxial MT wave feeder cable is connected to the matching device which is led into the body of the automobile through the conductive pipe which is used as the AM/FM wave antenna element and as the insulating members for the coaxial MT wave feeder cable. Polytetrafluoroethylene is used at least for the external insulation member covering the outer conductor.
With the above described structure the following effects result: because polytetrafluoroethylene is as low in specific inductive capacity as 2.07, even if the gap between the inner circumferential surface of the conductive pipe and the outer circumferential surface of the feeder cable is the same as in conventional cases, the stray capacity between the conductive pipe and the feeder cable becomes considerably lower in comparison to that of conventional cases. Also, polytetrafluoroethylene is excellent in heat resistance, blockage resistance, chemical resistance, buckling resistance, etc. and in addition it has the characteristic of being noticeably low in surface friction resistance. Consequently, it provides the advantages including that, in the case when the feeder cable is moved with the telescopic movement of the antenna element, even if it comes into contact with the draining member, the damage due to friction does not occur making it possible for the feeder cable to be used stabily for long time.